THE GASTROPOD GENUS YVANIA 15 



Class GASTROPODA 



Order ASPIDOBRANCHIA 



Family PLEUR< ITOMARIIDAE 



Genus YVANIA Bayle 



YVANIA INCLINATA Weller, 11. Sp. 

 PI. I, fi?.. 1; PI. II, fig. 9; PI. Ill, figs. 2-3b, 6 



Description. — Shell small, slightly turreted, spiral angle about 93°; 

 whorls angular, base convex; umbilicus small, partially closed; last whorl 

 forms over half the height of the shell ; surface marked by numerous revolv- 

 ing costae of various sizes; slit band prominent, concave, inclined slightly 

 to the upper sloping surface of the whorl and adjacent to the angle. 



The dimensions of the holotype are : height of entire shell 5.6 mm. ; 

 height of last whorl 3.4 mm. ; and width at base 5.4 mm. It is composed of 

 about five and a quarter whorls. The first two turns of the spire are uni- 

 formly convex and smooth. The third turn is also convexly rounded but it 

 is ornamented by eight fine equally prominent costae, five below and three 

 above the slit band which is outlined by two of them. A much finer costa 

 ap]>ears upon the band itself. Late in the third whorl the costae bounding 

 the slit band and lowest costa visible below the band become larger and more 

 prominent than the others and the ones above the band also become somewhat 

 more conspicuous than those below. In the course of the fourth turn the 

 mature characters of the shell are fully developed as exemplified by the angu- 

 lation and surface markings. 



The mature whorls are subrhomboidal in outline and are slightly wider 

 than high. The outer surface is divided into three parts by two angulations 

 the upi>er of which is sharply localized along the outer border of the slit 

 band and forms an angle of about 112°. situated below the suture by about 

 one fourth of the height of the whorl. The lower angulation is little more 

 than a regular inward curving of the shell from the flat lateral surface to the 

 convex basal surface of the whorl. The lateral surface makes up a little 

 more than one third the height of the whorl and slopes slightly outward from 

 a vertical position, as the diameter of the whorl at the lower angulation is a 

 little greater than at the upper. The basal surface is gently and nearly uni- 

 formly convex into the umbilicus. 



The whorls are practically non-embracing, as the spire is formed by each 

 succeeding whorl being attached to little more than the basal surface of the 

 preceding one. The suture is narrowly concave and follows the prominent 



